Friday, December 29, 2006

MJO, then El-Nino, or/then What?

Global tropical SSTs still remain above average, with the greatest anomalies across the Indo-Pacific region especially from the central equatorial Pacific to the west coast of South America. Magnitudes across the latter are ~plus 1-3C and extend to ~125m deep (up to 5C) per latest TAO buoy data. It appears that the oceanic Kelvin wave initiated around October 1st is about to reach the South American coast.

The warm-cool-warm SST distribution observed for the past several months has nearly disappeared suggesting our warm event may have already peaked (at least for this cold season). In any case, actual SSTs 29C (threshold for supporting sustained tropical convection) and higher extend from the South Pacific into eastern Indonesia and over portions of the Indian Ocean.

Since my last posting about a week ago, the most important development in the global weather-climate system has been the evolution of a coherent eastward propagating convectively coupled mode from the Indian Ocean into Indonesia. Per full disk satellite imagery the centroid of this feature was ~0/110E, and was clearly detached from lingering convection across the western Indian Ocean and South Africa (tropical cyclones understood). Interestingly portions of Indonesia that have been convectively suppressed for the last several months are finally getting enhanced rainfall. The Wheeler phase space plot and other tools suggest a strong projection onto a MJO for this eastward moving coupled tropical convective system. My own loose calculations have this MJO moving east ~6-7m/s (5 deg long/day), having speed up a bit during the last few days. I have a concern this MJO may shift into the west central Pacific faster than typically observed.

Forcing from the extratropics has led to yet another separate flare-up ~150-160E over the warm SSTs while the western hemisphere is dynamically suppressed. I do think this MJO will shift into the west central Pacific (while also drifting southward – most numerical and statistical tools agree), possibly consolidating with SST boundary forced convection farther to the east. I suspect we will also see the onset of the Australian monsoon as well as other seasonal cycle behaviors during the next 1-2 weeks. At some point (weeks 3-4?) enhanced convection would be expected to become quite strong around the equatorial date line and South Pacific as the MJO enhances the warm ENSO signal.

The global circulation has responded accordingly including anomalous twin upper tropospheric (baroclinic structures) Indian Ocean anticyclones and down stream west Pacific cyclones. Zonal mean anomalous easterly flow continues across the global tropical atmosphere and global relative AAM is slightly below normal (reanalysis climatology). However, equatorial upper tropospheric westerly wind anomalies (~5-15m/s) are starting to appear over the central and east Pacific. I think we are evolving from GSDM Stage 4 to 1. Complicating things have been the ~15 day central Pacific tropical convective flare-ups and rapid processes across the northern extratropics involving the east-Asian topography (mountain torque term of the global AAM budget).

As the MJO shifts into the west Pacific and eventually enhances the warm ENSO signal, subtropical westerly flow should increase (this process may already be starting per above). Also, as this tropical forcing excites Rossby wave trains which subsequently interact with extratropical baroclinic wave packets, I would expect anticyclonic circulation anomalies to begin dominating the northern polar latitudes and possibly lead to a warming of the stratosphere. In fact, per animations of 150mb daily mean vector wind anomalies, there have already been “bursting anticyclones” across the northern high latitudes for at least the past 1-2 weeks (and stratospheric temperatures have been rising). This would lead to negative phases of things like the AO.

As more models are showing, an evolution from GSDM Stage 1-2 is probable during the next 1-3 weeks. This means after week 1, ridge amplification around 140W into Alaska (linking up with a SSW signal?) may occur during week 2 with a downstream trough in the Rockies (~110W). This pattern would be expected to shift ~10 degrees to the east during week 3. At this time surface temperature anomalies of at least minus 10C are present across much of Alaska, and all that may eventually be forced southward into the western USA. Thus a cold and wet regime for much of the Rockies and Plains may be probable weeks 2-3 which may allow a good snow pack to build across large portions of the country. Afterwards (late January into February), a combined extended anomalously strong North Pacific jet ~30-35N leading to split flow across North America typical of a warm ENSO may be most probable (GSDM Stage 3). That would increase the risk of high impact precipitation events for the USA west coast, particularly California.

Finally, I offer a few comments. The intense tropical convective forcing that we have been observing across the Indian Ocean for the past several months is not typical of a warm ENSO (especially when it dominates over and suppresses the central Pacific). Also, there has been this issue of 2 regions of tropical convective forcing (Indian Ocean and west of date line) since early 2002. I think there a global warming signal (cause is unclear) being communicated to the atmosphere through the anomalously warm tropical SSTs. We have also been seeing lots of forcing-response-feedback scenarios involving the global oceans and atmosphere for the past several years which have been difficult to understand let alone predict. The point is there have been more GSDM “Stage 4-like” responses observed, and I suspect going into the February-April 2007 period we may see more occurrences Stages "4-1" global circulation behaviors.

Appendix

The following is a link to our recently accepted paper by MWR which discusses the GSDM (Weickmann and Berry 2006, in press).

In the beginning of Section 2 of that report we give a summary description of the Global-Synoptic-Dynamic Model (GSDM) of subseasonal variability.

I hope to get an update out by late next week. As stated previously, while I am at ESRL/PSD (starting 1/8/07) we hope to post at least short writings on this Blog every other day in support of the Hydrometeorological Testbed Project (HMT -- please see